A known constant velocity universal joint includes an inner race whose outer peripheral face is formed with inner grooves, an outer race whose inner peripheral face is formed with outer grooves, a plurality of balls each engaging with each pair of inner groove and outer groove, and an annular cage disposed between the outer peripheral face of the inner race and the inner peripheral face of the outer race. A multiple window portions are formed on the cage so as to penetrate therethrough for retaining the respective balls. Such the constant velocity universal joint is disclosed in Japanese Patent Laid-Open Publication No. 2000-46061. According to the constant velocity universal joint disclosed, when a rotation is input to the inner race, for example, the rotation of the inner race is transmitted to the outer race by means of the balls.
Then, when the rotation is transmitted from the inner race to the outer race, heat is generated since parts constituting the constant velocity universal joint are in contact with one another. In this case, by the decrease of coefficient of friction among the parts of the constant velocity universal joint, heat can be reduced according to the constant velocity universal joint disclosed. The reduction of heat among the parts results in prevention of flaking of the balls, and the like, which then leads to a long operating life of the constant velocity universal joint.
However, since the coefficient of friction among the parts is reduced, the sliding contact is caused between the inner groove with very high plane pressure and the ball, or between the outer groove and the ball. The sliding contact at a portion with very high plane pressure causes high abrasion, which leads to the decrease of the operating life of the constant velocity universal joint. Accordingly, occurrence of the sliding contact between the inner groove with very high plane pressure and the ball, and between the outer groove and the ball should be prevented. Further, the sliding contact should be changed to the rolling contact.
Thus, a need exists for a constant velocity universal joint in which a rolling contact can be obtained between an inner groove and a ball, and/or between an outer groove and a ball.